1. Field of the Invention
The invention relates to a polymer material having electro optic (EO) and nonlinear optical (NLO) properties suitable for manufacturing a device using EO and NLO properties. More particularly, the present invention relates to a polymer material having excellent EO properties and optical nonlinearities, maintaining the properties under the manufacturing conditions of high temperature, and having physical, chemical, and optical stabilities by introducing an organic chromophore as a side chain of the polymer so that the polymer material can be suitable for manufacturing an EO and NLO device, to a film manufactured from the polymer material, and to a method of manufacturing the polymer material.
2. Description of the Prior Art
Recently, as the optical devices for high speed and capacity data transmission have been rapidly developed, the demand for materials having EO and NLO properties suitable for that field has been rapidly increased and the associated research has been actively performed. Currently, inorganic crystal materials such as LiNbO3 and InGaAsP are used as materials of optical devices for optical communication suitable for high speed signal processing. These inorganic crystals have stable, high optical nonlinearity. However, every step of synthesizing the inorganic crystals is complicated and time consuming, which is increasing the manufacturing cost of the inorganic crystals.
On the other hand, many organic materials, particularly, polymer materials having EO and NLO properties have been developed for 20 years. The organic materials have advantages in their synthesis and producing processes in comparison to those of the aforementioned inorganic materials. Additionally, it is possible to control their properties such as production temperatures, refractive indexes, optical coefficients, and absorption wavelengths, etc., according to various requirements. These organic polymer materials having EO and NLO properties have prepared by reaction between the organic chromophores and polymers. The organic chromophores have conjugate linkages in their molecular structures for electron mobility and dipoles that are highly responsive to an external electric field due to the introduction of electron donating and electron accepting groups. The chromphores in a polymer matrix are aligned one directionally by applying an electric field, which is so called “poling”.
The efficiency of the poling is high at around the glass transition temperature (Tg) at which the molecular movement of the polymers is highly activated. A practical device process, particularly in electrode patterning, is generally carried out around 80–100° C. If Tg<150° C., there is an easy relaxation of the poled chromophores during the fabrication process. If Tg>200° C., it is very difficult to pole chromophores with very strong field at a temperature above 200° C. without dielectric breakdown. (M-H. Lee et al., “Polymeric Electro optic 2×2 Switch Consisting of Bifurcation Optical Active Waveguides and a Mach-Zehnder Interferometer”, IEEE J. on Selected Topics in Quantum Electronics, 7, 812, 2001). Therefore, it is preferable that the Tg of the polymer system for the EO and NLO devices is in the range of 150˜200° C.
Organic optical polymers are mainly classified into a host-guest polymer system, a side chain polymer system, a main chain polymer system, and a crosslinked polymer system (G. A. Lindsay, “Second-Order Nonlinear optical Polymers: An Overview”, ACS Symp. Ser. 60, G. A. Lindsay and K. D. Singer eds., ACS, 1995, chap. 1).
First of the types, the host-guest polymer system can be obtained by mixing an organic chromophore with a polymer matrix. The method is the simplest process. If the organic chromophore is well dispersed in the polymer matrix, the poling effect can be maximized due to increase in freedom of molecular motion of the organic chromophore. However, its optical nonlinearity is greatly reduced in the manufacture of optical devices at a high temperature due to its free molecular movement. And, as the amount of organic dye increases, the glass transition temperature (Tg) of the polymer system decreases, and light scattering occurs due to an agglomeration of organic chromophore molecules, resulting in optical loss. Secondly, the side chain polymer system is developed in order to solve the problems of the host-guest polymer system. In the side chain polymer system, an organic chromophore is chemically bound to a polymer to avoid the agglomeration of the organic chromophore and to provide the resulting polymer system with an appropriate Tg for the high stability of the optical nonlinearity. The side chain type polymer system can be obtained by introducing the organic chromophore into a side chain of the polymer.
Thirdly, the main chain polymer system can be obtained by incorporation of a nonlinear optical organic chromophore into a polymer main chain. As can be expected from this structure, the main-chain polymer system has lower molecular mobility than the side-chain polymer system and provides poor poling effect, but its optical non-linearity is thermally stable.
Finally, the crosslinked polymer system is used to improve the thermal stabilities of the host-guest polymer system and the side chain polymer system with low Tg Specifically, in order to improve the thermal stability of the optical nonlinearity of the NLO polymer after poling, the main chains of the polymer or the chromophores are crosslinked after the poling. By crosslinking reaction between the main chains of the polymer or between the chromophores, the movement of the organic chromophores is reduced so that the high optical nonlinearity can be maintained even at a high temperature. In general, the polymer main chain is thermo or photo cross-linked in the presence of a catalyst. However, after the cross-linking reaction, the unreacted cross-linkers or catalyst remain, which limits use of the cross-linked polymer system for optical devices (see U.S. Pat. No. 5,420,172 and U.S. Pat. No. 5,776,374).
Among the four types of polymer systems, the side chain type polymer system is known to be most suitable for the optical device in terms of the poling effect and the thermal stability of the optical nonlinearity. Especially, high Tgs of aromatic polyimides (PIs) have attracted attention due to stability in the aligned dipole orientation in the fields of EO and NLO polymers. In this case, since the PIs repeating unit has at least one functional group such as hydroxyl group and carboxyl acid group, the side chain EO and NLO PIs can be easily prepared from a reaction between the functional groups of the PIs and the organic ghromophores via the Mitsunobu reaction (O. Mitsunobu, “The Use of Diethyl Azodicarboxylate and Triphenylphosphine in Synthesis and Transformation of Natural Products”, Synthesis, 1, 1, 1981). (T-A. Chen et al., “Two-Step Synthesis of Side-Chain Aromatic Polyimides for Second-order Nonlinear optical”, Macromolecules, 29, 535, 1996; H.-J. Lee et al., “Synthesis and properties of nonlinear optical side chain soluble polyimides for photonics applications”, Journal of Polymer Science: Polymer Chemistry Edition 36, 301, 1998; E.-H. Kim et al, “Synthesis and characterization of novel polyimide-based NLO materials from poly(hydroxy-imide)s containing alicyclic units (II)”, Polymer, 40, 6157, 1999.; W. N. Leng et al., “Synthesis of nonlinear optical side-chain soluble polyimides for electro-optic applications,” Polymer, 42, 7749, 2001). This procedure is simple and reliable for the preparation of the side-chain EO PIs. In this reaction, however, the PI repeating unit must have at least one functional group such as a hydroxyl or a carboxyl acid group for the preparation of the side-chain EO PIs. The chemical structures of monomers for the preparation of the side-chain EO PI via the polymer reaction is limited for this reason.
Even though diamine monomers containing various EO chromophores have been recently developed for the preparation of the side-chain EO PIs, the preparation of the monomers is not easy and the purification process is needed for obtaining a high degree of polymerization (D. Yu et al., “Novel Aromatic Polyimides for Nonlinear optical”, Journal of The American Chemical Society, 117, 11680, 1995; U.S. Pat. No. 5,399,664). A tricyanovinyl group can be introduced into PIs with a specific chemical structure via tricyanovinylation for preparing EO and NLO PIs. In this case, the PIs chemical structure is also limited as in the Mitsunobu reaction (U.S. Pat. No. 5,688,906).